Lithographic plate and method of making same



United States Patent 3,277,820 HTHUGRAPHTC PLATE AND METHOD OF MAKING SAME John E. Pickard, Selma, Ind., assignor to Ball Brothers Company Incorporated, Muncie, Ind, a corporation of lfntliana No Drawing. Continuation of application Ser. No. 309,298, Sept. 16, 1963. This application Aug. 4, 1965, Ser. No. 477,296

6 Claims. (Cl. 101-1492) This application constitutes a continuation of my copending application, Serial No. 309,298, filed September 16, 1963, and now abandoned, entitled Lithographic Plate and Method of Making Same.

This invention relates to a new and improved lithographic plate and to a novel method of making the plate. More particularly, the invention relates to a new grained zinc lithographic plate and to a novel method for producing such graining.

Lithographic plates generally have been made from either zinc or aluminum sheets. Prior to the application of the photosensitive coating to the metal plate, it is customary to grain the plate so that a sharp uniform image can ultimately be achieved. While aluminum plates may be grained simply and conveniently through the use of a brush and/ or abrasive materials, the application of such methods to zinc sheets has not been completely satisfactory. The zinc plates are not grained as finely as aluminum plates with the result that although initial impressions made with such plates may be satisfactory, there is a tendency toward rapid deterioration of the images in subsequent impressions. In addition, such grained zinc plates oxidize readily which causes scumming of the non-image areas. As a result, the use of zinc lithographic plates has decreased appreciably even though aluminum plates have the serious disadvantage of relatively short press life. Thus, neither the aluminum plates nor the zinc plates allow long press runs.

In view of the difilculties and shortcomings of presently employed aluminum and zinc plates, it was completely unexpected and surprising to discover a new and improved zinc lithographic plate which does not possess the above deficiencies. Furthermore, the present invention provides a novel method which is simple and convenient and relatively low in cost. Thus, the invention provides a finely and uniformly grained zinc lithographic plate which heretofore was not attainable and provides a method for making grained zinc plates at a substantially reduced cost. In addition, the novel lithographic plate of the invention resists scumming and has substantially improved press life as compared with both the zinc and aluminum plates presently being employed.

In accordance with the present invention, a novel uniformly grained zinc lithographic plate is prepared by a method which comprises removing substantially all surface contaminants, oxides and scale from a surface of a zinc plate, subjecting said cleaned surface to a plurality of treatments with a sulfuric acid solution containing chromium trioxide, and removing substantially all of the acid from the plate after graining has taken place.

The zinc lithographic plate to be grained is first cleaned to remove substantially all of the surface contaminants, oxides and scales that may be present thereon. Advantageously, the plates are treated with hot water or steam and a caustic solution to remove any greasy or fatty contaminants on the surface. Preferably, the plate is rinsed with water to remove saponified grease and any residual caustic.

The removal of scale and oxide from the degreased plate may be facilitated through the use of an abrading or scrubbing operation. Oxide and/ or scale also may be removed by employing a sulfuric acid solution containing chromium trioxide and a water-soluble dichromate salt, with best results being achieved by employing the acid solution in combination with the scrubbing operation. After the scrubbing operation, the plate is preferably rinsed again prior to the graining treatment to minimize contamination of the graining bath.

The graining of the plate, as mentioned above, is produced by a plurality of treatments with a sulfuric acid solution containing chromium trioxide. The employment of a plurality of treatments rather than a single treatment expedites the graining process and produces more uniform graining of the plate. This is true even when the total time of the multiple treatments is significantly less than that of a single treatment. Advantageously, the bath is maintained at an elevated temperature, for example, in the range of about to 175 F. and preferably between about and F. Below about 75 F., the graining action is significantly reduced, while above about 175 F., there is a tendency for the graining to be non-uniform.

After the graining treatments are completed, the excess acid solution is removed from the plate. Advantageously, the surface is rinsed, and preferably the surface is scrubbed to insure that the acid solution is removed. If any of the acid solution is allowed to remain, the graining action may continue on the surface of the plate resulting in non-uniform graining and streaking of the plate.

The graining solution employed advantageously contains between about 1% and 25% by weight of sulfuric acid and between about 0.5% and 25% by weight of chromium trioxide. Preferably, the proportions of the sulfuric acid and chromium trioxide are each between about 2% and 20%, with a sulfuric acid content of about 7 8% to 15% and a chromium trioxide content of about 6% to 12% being especially advantageous. The total active ingredients advantageously are between about 2% and 50% by weight of the solution, with about 8% to 40% being preferred and about 12% to 25 being particularly advantageous.

The sulfuric acid solution employed for the cleaning and descaling of the plate advantageously contains between about 0.5% and 6% by weight of sulfuric acid, about 5% to 50% by weight of chromium trioxide and about 1% to 15% by weight of the dichromate. Preferably, the proportion of the sulfuric acid in the cleaning solution is between about 1% and 5%, the chromium trioxide between about 15% and 40% and the dichromate between about 3% and 11%. Particularly useful are solutions containing between about 2% to 4% sulfuric acid, 23% to 27% chromium troxide, and 5% to 9% dichromate. Advantageously, the total active ingredients in the solution should be between about 5% to 75% by weight, with about 10% to 60% solids being preferred and 15 to 50% solids being particularly advantageous.

The dichromate salt, as mentioned above, is a watersoluble salt with the alkali metal dichromates, e.g., the sodium and potassium salts, being preferred.

The following example illustrates one embodiment of the present invention and is not intended to restrict the scope of the invention. In the example, parts and percent of materials are by weight.

Example Rolled zinc plates of a size about 20 by 23 inches and a thickness of about 0.01 inch were immersed in tap water heated to a temperature of about F. vAfter about one minute, each plate was removed from the water bath and then immersed for about one-half minute in a 15% caustic soda solution. Thereafter, the plate was washed with tap water to remove saponified grease and residual caustic soda on the surface of the plate.

The washed plate was then scrubbed with a brush about 60 inches long, having bristles about 1% inches long, which was rotated at a speed of about 350 r.p.rn. As each plate was passed under the brush at a speed of about one foot per minute, a solution of about 25 parts of chromium trioxide, 7.5 parts of potassium dichromate, 3 parts of sulfuric acid and 64.5 parts of water was sprayed onto the surface of the plate. After the scrubbing treatment, the excess solution was removed from the plate with a squeegee.

The cleaned plate was rinsed again to remove any contaminants thereon and then immersed in an agitated bath containing about 8.5 parts of chromium trioxide, parts of sulfuric acid and 81.5 parts of water. After about .ten seconds, the plate was removed from the bath for about five seconds to permit the excess solution to drain from the plate, after which it was again dipped into the bath for a second treatment of approximately the same duration. The bath was maintained at about 110 F. during treatments.

After the second immersion, the plate was removed from the bath and permitted to drain for approximately forty-five seconds and then washed with water while the surface was scrubbed with a soft brush. After the washing step, the excess water was removed by passing the plate between squeegee rolls, and the plate was thereafter air dried.

All of the plates produced had very uniform, finely grained surfaces. A comparison of the plates made above with plates grained by mechanical methods shows that the chemically grained plates have a much higher degree of uniformity and cleanness of grain. In press life tests, the chemically grained plates gave substantially longer runs than those obtained with mechanically grained plates.

The above description shows that the present invention provides a novel and improved zinc lithographic plate and provides a simple and convenient method for producing the new plate. Moreover, the present invention provides a zinc lithographic plate having a highly uniform, finely grained surface which heretofore was not attainable. Furthermore, the method of the invention substantially reduces the cost of producing grained zinc plates of improved press life, both as compared with zinc plates and with aluminum plates presently being employed.

It will be apparent from the above description that various modifications in the procedures described in detail may be made Within the scope of the invention. Therefore, the invention is not intended to be limited to the specific procedures described herein except as may be required by the following claims.

What is claimed is:

1. A method of producing a zinc lithographic plate which comprises removing contaminants from the surface of the zinc lithographic plate, repeatedly subjecting the cleaned surface to treatment at a temperature between about 75 F. and 175 F. with an aqueous solution consisting essentially of water, between 1% and 25% by weight of sulfuric acid and between about 0.5% and 25 by weight of chromium trioxide until said plate is uniformly grained, and scrubbing the surface of the grained lithographic plate to remove substantially all residue after graining is completed.

2. A zinc lithographic plate which has been uniformly grained by the method of claim 1.

3. A method of producing a zinc lithographic plate which comprises removing scale, oxides and other contaminants from the surface of a zinc lithographic plate, repeatedly subjecting said clean surface to treatment at 5 a temperature between about 95 F. and 135 F. with an aqueous solution consisting essentially of water, between about 2% and 40% by weight of sulfuric acid and between about 2% and 20% by weight of chromium trioxide until said plate is uniformly grain, and removing substantially all residue from the lithographic plate promptly after graining is completed.

4. A method of producing a zinc lithographic plate which comprises removing scale, oxides and other contaminants from the surface of a zinc lithographic plate, repeatedly subjecting said clean plate to treatment at a temperature between about 75 F. and 175 F. with an aqueous solution consisting essentially of water, between about 8% and by weight of sulfuric acid and between about 6% and 12% by weight of chromium trioxide until said plate is uniformly grained, and removing substantially all residue from the lithographic plate soon after graining is complete.

5. A method of producing a zinc lithographic plate which comprises removing scale, oxides and other contaminants from a surface of the zinc plate by subjecting the surface to water at an elevated temperature, then to an aqueous caustic solution, and thereafter to an aqueous sulfuric acid solution containing about 5% to 50% by weight of chromium trioxide and about 1% to 15% by weight of a water-soluble dichrornate, repeatedly subjecting the resulting clean surface to treatment at a temperature between about 75 F. to 175 F. with another solution consisting essentially of water, about 1% to 25 by weight of sulfuric acid, and about 0.5% to 25% by weight of chromium trioxide until said plate is uniformly grained, and removing substantially all residue from the lithographic plate promptly after graining is complete.

6. A method of producing a zinc lithographic plate which comprises subjecting the surface of a zinc lithographic plate to water at an elevated temperature; treating said plate with about a 15% aqueous caustic soda solution; rinsing said caustic soda solution and surface contaminants from said plate with water; scrubbing said plate while subjecting said plate to an aqueous sulfuric acid solution containing about 5% to 50% by weight of chromium trioxide and about 1% to 15% by weight of a water-soluble dichromate; rinsing said plate with water; subjecting said cleaned surface to a plurality of treatments with a second aqueous solution consisting essentially of water and substantially equal proportions between 2% and by weight of sulfuric acid and chromium trioxide at a temperature between 75 F. and 175 F.; scrubbing said plate while rinsing said plate with water; and drying said rinsed plate.

References Cited by the Examiner UNITED STATES PATENTS DAVID KLEIN, Primary Examiner. 

1. A METHOD OF PRODUCING A ZINC LITHOGRAPHIC PLATE WHICH COMPRISES REMOVING CONTAMINANTS FROM THE SURFACE OF THE ZINC LITHOGRAPHIC PLATE, REPEATEDLY SUBJECTING THE CLEANED SURFACE TO TREATMENT AT A TEMPERATURE BETWEEN ABOUT 75* F. AND 175* F. WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF WATER, BETWEEN 1% AND 25% BY WEIGHT OF CHROMIUM TRIOXIDE UNTIL SAID PLATE IS UNIBY WEIGHT OF CHROMIUM TRIOXIDE UNTIL SAID PLATE IS UNIFORMLY GRAINED, AND SCRUBBING THE SURFACE OF THE GRAINED LITHOGRAPHIC PLATE TO REMOVE SUBSTANTIALLY ALL RESIDUE AFTER GRAINING IS COMPLETED.
 2. A ZINC LITHOGRAPHIC PLATE WHICH HAS BEEN UNIFORMLY GRAINED BY THE METHOD OF CLAIM
 1. 